1. Field of the Invention
The present invention relates to a circuit, and more particularly, to an electrostatic discharge (ESD) protection circuit.
2. Discussion of the Related Art
A conventional ESD protection circuit will be explained below with reference to the accompanying drawings. FIG. 1A shows an ESD protection circuit located inside a conventional device, i.e., a conventional internal ESD protection circuit. FIG. 1B shows an ESD protection circuit located outside of a conventional device, i.e., a conventional external ESD protection circuit. The conventional ESD protection circuits perform the function of preventing the associated device from being destroyed due to internal or external static electricity.
Referring to FIG. 1A, the internal ESD protection circuit is formed as a first part, i.e., at a first stage, of an internal circuit of a device. The ESD protection circuit includes a first diode 1 and a second diode 2. The anode of the first diode 1 is connected to a positive (+) power voltage (Vdd) and its cathode is connected to input/output ports (or pad) of the internal circuit. The anode of the second diode 2 is connected to the input/output ports of the internal circuit to which the cathode of the first diode 1 is connected, and the cathode of the second to diode 2 is connected to a negative (-) power voltage Vss.
The first diode 1 protects the internal circuit from positive static electricity, and the second diode 2 protects the internal circuit from negative static electricity. The first and second diodes 1 and 2 are PN junction diodes. It is also possible to use transistors instead of the diodes.
The external ESD protection circuit shown in FIG. 1B has essentially the same structure as the external ESD protection circuit shown in FIG. 1A. The only difference is that the external ESD protection circuit in FIG. 1B is located outside the device. Similarly, the first and second diodes 1 and 2 are PN junction diodes. Again, it is possible to use transistors instead of the diodes in the external ESD protection circuit, shown in FIG. 1B.
FIG. 2A shows an internal ESD protection circuit employing transistors, and FIG. 2B shows an external ESD protection circuit employing transistors. That is, instead of the diodes shown in FIGS. 1A and 1B, a diode connection of a PMOS transistor (DPT) and a diode connection of a NMOS transistor (DNT) are used. Here, the DPT replaces the first diode 1 of FIGS. 1A and 1B, and the DNT replaces the second diode 2 of FIGS. 1A and 1B.
The operation of the conventional ESD protection circuit will now be explained. Since the internal ESD protection circuit and the external ESD protection circuit operate similarly, only the operation of the internal ESD protection will be explained. Referring to FIG. 1A, when positive-type static electricity is applied through a pad, the potential at a node A is increased. If the potential at the node A becomes higher than the positive power voltage Vdd by a value equal to the turn-on voltage Von of the first diode 1 due to the applied static electricity, the first diode 1 is turned on. Accordingly, if the potential at node A becomes at least Vdd+Von, the static electricity is by-passed through the first diode 1. Thus, the voltage above Vdd+Von is not transmitted to the internal circuit.
On the other hand, when negative static electricity is applied through the pad, the potential at the node A is decreased. If the potential at node A becomes lower than the negative power voltage Vss by a value equal to the turn-on voltage Von of the second diode 2, the second diode 2 is turned on. Accordingly, if the potential becomes lower than or equal to Vss-Von, the static electricity is by-passed through the second diode 2. Thus, the voltage below Vss-Von is not transmitted to the internal circuit. The ESD protection circuits of FIGS. 2A and 2B employing transistors instead of diodes operate the same way.
The conventional ESD protection circuits have the following problems. If the internal resistor of the diode is destroyed due to an excessive static electricity applied instantaneously, the internal circuit cannot be protected from the static electricity. Thus, the size of the diode or transistor has to be large. This makes the device high-integration and miniaturization impossible.